A total of 1132 samples of maternal milk expressed from the disinfected breast on the first days after parturition were subjected to microbiological examination. Contamination by group B streptococci (Streptococcus agalactiae) was demonstrated in 40 samples (3.53%). Strains carrying the antigens Ia, Ia/c, Ib/c, II, III and R were represented among the isolates. Type Ia/c was the commonest; antigens II and III were always combined with the R antigen. The milk was either very strongly contaminated with almost pure culture demonstrable by direct cultivation of the milk sediment, or only isolated colonies were obtained in primary culture, often not until after enrichment. If the first samples gave dense growths, second-sample cultures were also positive. The strains isolated from samples of the individual mothers milk differed by their antigenic type structure; this gave evidence that the infection was not of nosocomial origin. Immunoglobulin levels in the whey of positive samples were not different from the whey of mothers not shedding group B streptococci. The health of both mothers and infants did not deviate from the general average for normal mothers and infants. No signs common to all the mothers shedding group B streptococci in their milk and no marked effect on the infants' health up to the age of 1-2 years were established.